1. Field of the Invention
The present invention relates generally to methods for determining material properties by microscopic scanning, and more particularly relates to determination of material properties including dopant profiles of materials using capacitance-voltage techniques.
2. State of the Art
With the shrinkage of semiconductor devices to the sub-micron level, a great need exists for direct, quantitative, two dimensional dopant profile measurement on a nanometer scale. Direct measurement of two dimensional dopant profile provides a means for improving very large scale integrated process and electrical device simulators. Direct two dimensional profile measurements are needed to accurately calibrate and verify models built into these simulators. Additionally, direct two dimensional measurements can also give rapid feedback to improve control of manufacturing processes and decrease process development time.
There are many one dimensional dopant profiling techniques presently available, such as secondary ion mass spectroscopy, spreading resistance, junction staining and anodic sectioning. These techniques do not provide quantitative two dimensional dopant profile information. The advent of the scanning tunneling microscope and the more recent scanning probe microscope have provided a new direction for direct device characterization on a nanometer scale. The scanning capacitance microscope has, in particular, shown great potential for direct measurement of two dimensional dopant profile with nanometer scale spacial resolution.
The measurement of local dopant density was disclosed in U.S. Pat. No. 5,065,103 which issued on Nov. 12, 1991. The general method of measuring local capacitance and a generic inversion algorithm are disclosed in this patent. Also disclosed is the use of an atomic force microscope to position the probe at the surface of the semiconductor sample that is to be characterized.
Since the issuance of U.S. Pat. No. 5,065,103, it has been found that the inversion of scanning capacitance microscope data to dopant profile is extremely difficult by the methods described in that patent. In fact, it has not been possible to achieve a quantitative agreement between scanning capacitance microscope measured profiles and the actual dopant profiles, at least when using a one dimensional model as a basis for determining dopant density profiles from the data produced by the scanning capacitance microscope.
The source of the problem to achieve quantitative inversion is that as the tip of the scanning capacitance microscope moves from a region of high dopant density to one of low dopant density, the amount of depletion of majority carriers beneath the tip due to voltage applied between the tip and the sample varies significantly. In regions of high dopant density, the depth of depletion of majority carriers may be small compared to the tip radius. In regions of lower dopant density, the depth of depletion of majority carriers may be comparable or larger than the tip size. In order to quantitatively invert the scanning capacitance microscope data under these conditions, a three dimensional model of the tip/sample interaction is required. Since the exact size and shape of the tip are typically unknown, inversion based upon the three dimensional model is very difficult. It would be highly desirable to provide a method of using a scanning capacitance microscope in which the data obtained from the microscope could be quantitatively inverted to dopant profile using a simplified one dimensional model as a basis for the inversion.